The present invention relates to centrifugal concentrators of the rotating bowl type and other enhanced gravity machines for the separation of solids of higher density such as gold, iron or tin from a slurry containing solids of a lower density and liquid and more particularly such machines in which the target concentrate is continuously discharged.
Various centrifugal concentrators and other enhanced gravity machines are known which separate particles of high density such as gold, iron or tin from tailings and other slurry streams in a manner whereby the concentrate is discharged continuously rather than in a batch process requiring periodic shutdown of the machine. Generally such concentrators have utilized pinch valves or fixed orifice spigots to control the release of the concentrate from the rotating machine. For example, the present inventor""s continuous discharge centrifugal concentrator which is the subject of U.S. Pat. No. 5,462,513 issued 31 Oct. 1995 utilizes flow control valves to control the discharge of concentrate which are air controlled mini pinch valves constructed with sleeves of the type manufactured by Linatex Inc. Each mini pinch valve has a central bore in which is positioned the flexible cylindrical sleeve of abrasion resistant material. By applying air pressure to the exterior surface of the sleeve, the sleeve is compressed and closes off the central bore, preventing the passage of concentrate. When air pressure to the valve is reduced the sleeve opens and material may flow through the valve. Another continuous discharge separator is disclosed in Knelson U.S. Pat. No. 5,338,284 issued Aug. 16, 1994. That device similarly utilizes a standard pinch valve to control the flow of discharged concentrate. Similarly, in the continuous discharge separator disclosed in Knelson U.S. Pat. No. 5,601,523 issued Feb. 11, 1997, pinch valves are used to control the flow of discharged concentrate.
The use of standard pinch valves to control discharge of concentrate from enhanced gravity machines carries with it a number of problems. In a pinch valve, the circular sleeve is typically compressed between two planes, causing the cross-section shape of the sleeve to be flattened, without reducing the circumference of the orifice. When the opening in the valve is reduced to reduce flow, the flattened cross-section thus created tends to trap coarse particles which can quickly cause the passage to become blocked, and can only be dislodged by fully opening the valves. Also, the sleeves are more easily torn by coarse particles when stretched and under tension. Alternatively, the valve can be operated in an on/off mode. This creates more problems. First, if this technique is to be effective, the valve needs to be cycled very rapidly, which causes failure after a few hundred hours of operation. Second, on/off cycling creates discrete bursts which may allow valuable material to bypass and cause barren material to be captured.
Other types of concentrators which provide a continuous discharge of concentrated fractions through small spigots having fixed orifices are the xe2x80x9cKelsey jigxe2x80x9d disclosed in Kelsey U.S. Pat. No. 4,454,041 issued Jun. 12, 1984, and U.S. Pat. No. 4,898,666 issued Feb. 6, 1990; and the xe2x80x9cCampbell jigxe2x80x9d disclosed in Campbell U.S. Pat. No. 4,279,741 issued Jul. 21, 1981, and U.S. Pat. No. 4,998,986 issued Feb. 6, 1990. In such machines, is desirable to minimize the amount of water flowing out the concentrate discharge by minimizing the diameter of the spigot orifice. However this leads to blockage of the orifice by coarse particles, which causes imbalance in the rotor and requires shut-down of the machine.
Flow control valves of the type called xe2x80x9cradially constrictible unobstructed venturi valvesxe2x80x9d have been used in the past in pipelines. A particular type of these valves, called xe2x80x9cmuscle valvesxe2x80x9d have been developed by The Clarkson Company of Reno, Nevada for use as low-pressure throttling control valve in pipeline systems. The basic design of such flow control valves is disclosed in U.S. Pat. No. 3,090,591 issued May 21, 1963. Such valves have not previously been used in rotating systems or gravity enhanced concentrators where high pressures are encountered. Unlike pinch valves, they utilize a xe2x80x9cmusclexe2x80x9dxe2x80x94a rubber part which uniformly constricts the sleeve so that as the sleeve diameter is reduced it maintains a circular cross-section.
There is therefore a need for a continuous discharge centrifugal concentrator having flow control valves which have the advantages of xe2x80x9cmuscle valvesxe2x80x9d.
The present invention provides, in an enhanced gravity machine for separating particulate material of higher specific gravity from particulate material of lower specific gravity, comprising a) a rotating member adapted for rotation about an axis, (b) material supply means to deliver the particulate material into the rotating member, c) a plurality of cavities extending outwardly with respect to the axis of rotation of the rotating member, the cavities each having an outlet, and d) flow controlling means for controlling the flow of material from the outlets of the cavities; the improvement wherein the flow control valves are adapted to provide an orifice of continuously variable perimeter over a substantial range of operating cross-sectional areas.